Cold, hard concentration
As standard waste water treatment systems become overloaded, and the drive for the "zero-discharge" plant becomes ever-greater, new methods of treatment have to be found. Steve Holt, Niro Ltd, looks at the role of freeze concentration as a practical and cost-effective way of meeting legislative requirements and protecting the environment.
Niro Freeze Concentration (NFC) is a process by which wastewater can be concentrated to greatly increase the solids content, thereby significantly reducing the power required in incineration. The process works by ³growing² pure ice crystals in the feed solution, then extracting and melting them to provide a clear flow of pure water (<50ppm) that may be reused or disposed of safely. The remaining concentrate has a much higher solids content and can be destroyed through conventional incineration at much lower cost. The process also offers additional savings compared with thermal concentration or oxidation processes which must use expensive materials due to the higher process temperatures.
In its simplest form, the unconcentrated liquid is pumped from a feed tank through a heat exchanger, which instantly forms small ice crystals. These crystals are pumped to a recrystaliser where they are mixed with larger ice crystals. The small crystals have a slightly lower equilibrium temperature than the larger crystals and so melt on the surface of the larger ones then refreeze causing the large crystals to grow further.
When the crystal formation process is complete, the resulting slurry is pumped to a wash column where the ice is separated and discharged leaving the concentrated solution behind for incineration. As crystallisation creates "pure" water crystals the residual solids in the water is very low.
The closed system design eliminates vapour/liquid interfaces and, therefore, prevents volatile losses from the system. Water produced in this way will meet most quality requirements for direct discharge. It can be used directly in the plant without the need for further bio-treatment.
Freeze concentration provides a stable operation base due to the relatively large volume of ice in the system. Changes in the feed are absorbed in the system with little effect on the water removal capacity and with no operator involvement.
The NFC process has been successfully tested on waste solutions containing various compounds such as: sodium hydroxide, sodium benzoate, sodium acetate, magnesium sulphate and other organic and inorganic salts. It will also treat solutions containing acetaldehyde, ethanol, methanol and more.
The process has the benefit of being able to recover water from solutions without any loss of the compounds themselves. In addition, the system requires little routine maintenance, is simple to operate and is insensitive to variations in feed composition. Capacity ranges from 500 to 50,000kg/hr are available on a commercial scale.
NFC is already accepted as proven technology within the food industry for concentrating fruit juices, etc. It is now beginning to be recognised as appropriate for wastewater treatment and has significant benefits over existing evaporation and oxidation processes.
There are already two commercial plants up and running: one in Singapore for Seraya Chemicals (a Shell/Mitsubishi/Sumitomo/Phillips Petroleum joint venture) which has operated since 1997; and one in the Netherlands for Basell (a BASF/Shell joint venture) which is now in the commissioning phase.
The SCSL plant uses the system for disposing of the liquid waste stream from their styrene monomer/propylene oxide (SMPO) production facility. The primary organic hazardous components (POHCs) are considered to be hazardous to the environment and are destructive to standard bio-treatment systems.
Shell performed an extensive feasibility study during 1991-1994 of a
of disposal options including various wet-air oxidation processes and
evaporation and freeze concentration followed by incineration and
biotreatment. the capital and operating costs of freeze concentration were
similar to others in the study. However, when all the auxiliary components,
specialised construction, extensive pilot plant and validation work, and
known operational problems were included for each method, freeze
concentration came out on top, meeting all the plant¹s water discharge,
operating service factor, process stability and control, and economic